One of silicone rubber's most important characteristics is its chemical inertness. Composed primarily of polydimethylsiloxane chains, crosslinked into a solid thermoset elastomer, ordinary silicone rubber is particularly resistant to both chemicals and heat [1] [2]. The stability of silicone rubber is useful in seals, medical devices, and many industrial and consumer applications requiring elastic materials that are unaffected by environment [1]. If something spills on silicone rubber, such a spill can usually be removed by just wiping the spill off the silicone rubber.
Unfortunately, the relative inertness of silicone rubber also makes silicone rubber incompatible with most adhesives and coatings; i.e. most adhesives and coatings peel away easily after application. Silicone rubber has a low-energy surface [3] that is non-polar, non-porous, and lacking in reactive groups, so the silicone rubber surface forms only weak molecular bonds with most materials that try to adhere to the silicone rubber. Generally, glues will not adhere to silicone rubber and paints will not stay on the silicone rubber surface. There are, however, two important exceptions to that non-adhesion rule.
First, silicone adhesives and coatings can adhere to silicone rubber [4]. The high mobility of polydimethylsiloxane molecules permits an uncured reactive silicone fluid to dissolve into a silicone rubber [5] and then crosslink. The resulting interpenetrating polymer network (IPN) binds a first silicone solid to a second silicone solid [6]. Unfortunately, while silicone adhesives and coatings are usually effective on silicone rubbers, silicone adhesives and coatings do not adhere well to many other materials. Furthermore, silicone adhesives and coatings typically cure slowly, often require moisture while curing, have limited cohesive strengths, and may exhibit cure incompatibilities [5] [7].
Second, cyanoacrylate adhesives can adhere to silicone rubber that has been primed with molecules that initiate polymerization of the cyanoacrylate monomer [8] [9] [10]. As cyanoacrylate molecules diffuse into the primed silicone rubber, the cyanoacrylate molecules encounter initiator molecules and form polycyanoacrylate chains that are inextricably entangled with the silicone rubber's polydimethylsiloxane chains. When pressed between two substrates, one or both of which are primed silicone rubber, the cyanoacrylate adhesive hardens in seconds into a rigid or stiff plastic that binds the two substrates firmly together.
Cyanoacrylate bonding of silicone rubber is not without potential problems. Because the bond forms in seconds and is not preceded by preliminary tack or green strength, proper alignment can be difficult. There is frequently a mechanical mismatch between the firm, inextensible polycyanoacrylate bond line and the soft, elastic silicone rubber it bonds. Even with rubber-toughened or semi-flexible cyanoacrylates, the polycyanoacrylate bond line is not an elastomer and imposes its own characteristics on the materials it joins. That bond line will fragment if the materials it joins are stretched significantly.
With such limited choices for bonding and coating silicone rubber, alternative approaches would be valuable. Additionally, low-energy surfaces such as HDPE, LDPE, PTFE, silicones, and many elastomers are notoriously difficult to glue or paint. Most adhesives, inks, coatings, and paints form only weak bonds to low-energy surfaces. Those adhesives and coatings that do bind well to low-energy surfaces are limited and limiting.
There is a long felt need in the art for compositions and methods useful for applying adhesives and coatings to surfaces that do not typically bond well to adhesives and coatings.